Assertion: Carbon reacts with oxygen to form carbon dioxide which is acidic oxide.

Reason: Non-metals form acidic oxides.

Both Assertion and Reason are true, and Reason is the correct explanation of the Assertion.

Assertion: When carbon burns in the presence of oxygen, carbon dioxide ($C{O}_2$), an acidic oxide, is created. Hence carbon in its aqueous solution turns blue litmus into red. 
$C+ O_2\to C{O}_2$

Reason: Non-metallic oxides have an acidic character. When dissolved in water, they produce an acidic solution that turns the blue litmus solution red. Carbonic acid (${H_2{\mathit{CO}}_3}_{ }$) and sulphuric acid, for instance, react with water when non-metal oxides like ${\mathit{CO}}_2$ and ${\mathit{SO}}_2$ are present ($H_2{\mathit{SO}}_4$).